Clothing insulation in a hypobaric environment.

Hypothesis: Clothing insulation is the result of complex interactions between heat transfer mechanisms and clothing material thermal resistances. Hypobaria changes the heat transfer processes therefore should have observable effects on the clothing insulation. Methods: The effect of hypobaria on the thermal insulative properties of U.S. Army fatigue uniform (BDU) and U.S. Army chemical protective overgarment (BDO) were examined. Barometric pressure of 429 mmHg, comparable to the condition at terrestrial elevation of 4570 m (15,000 ft) above sea level was created in a hypobaric chamber. The sea level environment was used as a baseline condition. Results: Our data support a diminished convective heat transfer and an enhanced evaporative heat transfer at higher altitude. We also found that hypobaria had only a small effect on the intrinsic clothing insulation values. For the less insulative BDU, hypobaria did not appreciably affect clothing insulation values. For the more insulative BDO, a maximum difference of 0.2 clo (clo = 0.155 m 2 .K.W -1 ) was found between hypobaric and normobaric environments. Conclusion: Heavy clothing insulation forced the heat transfer processes at the skin surface to operate almost independently from those at the clothing surface. At the skin surface, evaporation was the dominant process, while at the outer clothing surface, convection dominated. At higher altitude, enhanced evaporative heat transfer resulted in a lower skin temperature, while reduced convective heat transfer hampered heat dissipation from clothing surface to the ambient environment, hence elevating the clothing temperature. Therefore, in hypobaric environment, the skin temperature was found to be lower, but the clothing temperature higher than at sea level.